1. Field of the invention
The present invention relates to a camera having a zoom lens in which at least one lens group is driven to move by a cam mechanism to vary a focal length of the zoom lens.
2. Description of the Related Art
In a conventional zoom lens of a camera, at least one lens group (movable lens group), in a plurality of lens groups aligned along a common optical axis, is driven to move in the optical axis direction to vary the focal length of the whole optical system. A cam mechanism is often used to move the movable lens group in the optical axis direction. Namely, the movable lens group is driven to move through a cam, generally in the form of a cylinder or plate, which is moved manually or is motor-driven. The cam is provided on its surface with at least one cam edge, and a groove or slot in which a follower fixed to the movable lens group is engaged, so that the movable lens group is moved in response to the movement of the cam. Such a cam Mechanism is effectively utilized in a case where two or more than two movable lens groups are moved relative to each other in a predetermined relationship to effect zooming.
In the case where the cam is in the form of a cylinder, i.e., the cam is provided as a cam cylinder, the cam cylinder is arranged around the movable lens group concentrically to the whole zoom lens optical system so that the movable lens group is moved by a rotational movement of the cam cylinder. On the other hand, in the case where the cam is in the form of a plate, i.e., the cam is provided as a cam plate, the cam plate is arranged aside of the movable lens group to be movable in a direction perpendicular to the optical axis direction of the zoom lens optical system, so that the movable lens group is moved by a linear movement of the cam plate. As a type of cam plate a disk-shaped cam plate is known. In the case of such a disc-shaped cam plate being used, the disc-shaped cam plate is rotated so as to move the movable lens group.
In the case of the aforementioned cam cylinder being used, because the cam cylinder is arranged around the movable lens group, the diameter or height of the zoom lens is unavoidably made large, which is disadvantageous for designing a camera having a short height. In the case of the aforementioned cam plate being used, a large space for the linear movement of the cam plate is necessary to be provided in a camera body, which makes it difficult to design a slim camera. This is also true in the case of the aforementioned disk-shaped cam plate being used.
Instead of using the cam mechanism, the movable lens group can be moved in the optical axis direction using a lead screw to effect zooming. In this case, the lead screw is arranged aside of the movable lens group parallel to the optical axis thereof. A female thread portion formed in a supporting frame of the movable lens group is engaged with the lead screw, so that the movable lens group is moved in the optical axis direction when the lead screw rotates. The lead screw is generally rotated by a motor. It is necessary to provide one lead screw and one motor when one movable lens group needs to be moved to effect zooming. However, when more than one movable lens group needs to be moved to effect zooming, a corresponding number of lead screws and motors need to be provided. Thus, the structure of a lens driving mechanism becomes large and complicated and accordingly it is difficult to design a slim camera.